1. Field of the Invention
The invention relates to a bone fixation device according to the preamble of claim 1. It relates in general to the field of medical devices used to treat bone fractures. In particular, it relates, but is not limited, to a bone screw with features and/or attributes to provide a variable fixation to bone fragments during the fracture healing period.
2. Description of the Related Art
The skeleton responds to changes in strain adjusting its bone mass and distribution in order to restore the optimal local strain level. Under favorable boundary conditions, a bone fracture is followed by either the primary or secondary repair process in which the anatomical and functional skeletal element capability is restored through different phases. These repair processes as well are modulated by changes in the mechanical environment.
Since more than 50 years, bone fracture treatment is routinely carried out through intramedullary, extramedullary internal or external load carriers such as, for example, nails, bone plates, etc., when necessary used in combination with fixation elements like in e.g. bone screws in order to provide to the bone fragments the stability promoting one of the repair processes. Later on, a variety of bone fixation devices featuring the association of a resorbable/degradable and a non-resorbable material have been invented to progressively load at a late stage in the healing process the treated bone/bone fragments taking advantage of the molecular weight decrease, reduction in mechanical strength and finally mass loss phases a resorbable/degradable material undergoes once implanted. The aim was stress shielding decrease (U.S. Pat. Nos. 4,756,307A, 4,655,203A, 4,338,926, 4,773,406A, 5,013,315A), or to vary the stability among the different parts of a compound (U.S. Pat. No. 8,506,608, US2007233073A1, US2008317812A1) or composite medical device (US2012029564A1).
From U.S. Pat. No. 8,506,608 CERYNIK ET AL. an orthopedic screw for use in a fixation device for treating fractures is known. The known screw has a sleeve of bioresorbable material surrounding the screw shaft and fixed thereto, whereby the diameter of the sleeve is larger than the diameter of the shaft.
The aim of CERYNIK ET AL. is to change the properties of the compound fixation device through modifying the performance of the plate to screw connection, namely to change the locking mechanism between screw and plate to become a non-locking mechanism over time. The function of gradually bringing additional strain to the callus just under the plate is not purposeful. With the polymer resorption the plate/screw construct goes from locking to non-locking and therefore the positive aspects of the locking stability are lost for the remaining period of treatment.
There remains therefore the need for a device for bone fixation allowing decreasing gradually the stiffness of the bone-implants construct without losing the benefit of the locking fixation until the bone plate is removed. Lately, some fixation elements have been invented in order to provide through dynamic or far cortical locking a flexible engagement to the fractured bone fragments (U.S. Pat. No. 8,114,141B2, US2006195099A1, US2012029579A1) at an early stage of fracture treatment. This dynamisation or additional flexibility is fully provided at the beginning of the fracture treatment and doesn't change during the medical treatment.